Molecular Aspects of Secretory Granule Exocytosis by Neurons and Endocrine Cells

Neuronal communication and endocrine signaling are fundamental for
integrating the function of tissues and cells in the body. Hormones
released by endocrine cells are transported to the target cells
through the circulation. By contrast, transmitter release from
neurons occurs at specialized intercellular junctions, the
synapses. Nevertheless, the mechanisms by which signal molecules
are synthesized, stored, and eventually secreted by neurons and
endocrine cells are very similar. Neurons and endocrine cells have
in common two different types of secretory organelles, indicating
the presence of two distinct secretory pathways. The synaptic
vesicles of neurons contain excitatory or inhibitory
neurotransmitters, whereas the secretory granules (also referred to
as dense core vesicles, because of their electron dense content)
are filled with neuropeptides and amines. In endocrine cells,
peptide hormones and amines predominate in secretory granules. The
function and content of vesicles, which share antigens with
synaptic vesicles, are unknown for most endocrine cells. However,
in B cells of the pancreatic islet, these vesicles contain GABA,
which may be involved in intrainsular signaling.' Exocytosis of
both synaptic vesicles and secretory granules is controlled by
cytoplasmic calcium. However, the precise mechanisms of the
subsequent steps, such as docking of vesicles and fusion of their
membranes with the plasma membrane, are still incompletely
understood. This contribution summarizes recent observations that
elucidate components in neurons and endocrine cells involved in
exocytosis. Emphasis is put on the intracellular aspects of the
release of secretory granules that recently have been analyzed in
detail.